A silicon carbide (i.e., SiC) semiconductor device may have a MOS structure similar to a silicon semiconductor device. For example, US Patent Application Publication No. 2007/0045631-A1 teaches a vertical power MOSFET as a SiC semiconductor device. This MOSFET includes an interlayer insulation film made of a LTO film, which is formed on a surface of a gate electrode. In this case, the LTO film may have a crack or a failure in shape, so that an electrode wiring disposed on the LTO film may be disconnected or the electrode wiring and the gate electrode may short-circuit. Accordingly, the MOSFET may have a gate leakage failure. Therefore, the interlayer insulation film is made of a BPSG insulation film, which is also used for the silicon semiconductor device. In such a case, by performing an anneal process, a boron softening effect generates so that the crack failure and the failure in shape are prevented.
In the vertical power MOSFET, a source electrode as an upper electrode contacts a N+ source region with ohmic contact. The source region is made of N type semiconductor. Thus, the source electrode is made of material having ohmic contact with the N type semiconductor, for example, made of Ni. Therefore, the material such as Ni is diffused in the BPSG insulation film.
The inventors preliminarily studied diffusion of a Ni component in a Ni source electrode into a BPSG insulation film, as a related art. FIG. 7 shows a result of SIMS analysis of a Ni concentration in the BPSG insulation film. The Ni atoms are diffused from a contact portion between the BPSG insulation film and the Ni source electrode.
When a component of the source electrode is diffused in the BPSG insulation film, the insulating property of the BPSG insulation film is reduced. For example, FIG. 8 shows a relationship between a drain voltage VD and a drain current ID in case of Ni diffusion in the BPSG insulation film and in case of no diffusion in the BPSG insulation film. VIIIA represents a case where the Ni atoms are diffused in the BPSG insulation film, and VIIIB represents a case where the Ni atoms are not diffused in the BPSG insulation film. When the Ni atoms are diffused in the BPSG insulation film, the drain current ID rapidly increases at the drain voltage VD of 200 volts. Accordingly, the insulation breakdown voltage is about 200 volts, which is much smaller than that in a case where the Ni atoms are not diffused in the BPSG insulation film. When the Ni atoms are not diffused in the BPSG insulation film, the drain current ID rapidly increases at the drain voltage VD of the 700 volts. Thus, the insulation breakdown voltage is about 700 volts. To reduce the influence of the Ni diffusion in the BPSG insulation film, it is considered that a thickness of the BPSG insulation film is increased. However, in this case, a process time in a forming step of the BPSG insulation film becomes longer. Further, an etching time in a step for forming a contact hole in the BPSG insulation film becomes longer. Accordingly, it is preferable to reduce the influence of the Ni component diffusion in the BPSG insulation film having a small thickness.